[c-nsp] FW: Overruns

[Nick Hilliard]

On 27/01/2011 07:57, Mohammad Khalil wrote:
> its on Cisco 7606-S , the connection is port channel with 5 physical interfaces

Oh, you Really Don't Want To Do That(tm).  For etherchannels on EARL7 
architecture, if you want your load balancing to be roughly equal, you need 
to ensure that your port channels are configured with either 2, 4 or 8 
physical interfaces.  The reason for this is due to limitations on the 
EARL7 chip on the sup720 - specifically, there are only 3 bits of bucket 
space, which means 1) no more than 8 active links and 2) severe limitations 
in the load balancing algorithm.

If you have 5 physical interfaces, the load balancing will work out (in the 
optimal case) as 2:2:2:1:1.  This means that you effectively have 
(2+2+2+1+1)/(2+2+2+2+2) = 4/5 of the total etherchannel capacity available 
for traffic.  I.e. you're actually not gaining anything by using more than 
4 physical links.

There's a description here:

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml#cat6k

As this is a hardware limitation, the same load balancing profiles will be 
seen on IOS.  So you can see from the table that even if you configure 
extra ports in a port channel interface, the overall bandwidth will not 
actually increase except in the case of 3 bearer links, where the 
aggregated bandwidth will be 2.66 * the speed of an individual physical link.

Incidentally, I stand corrected on Mikael Abrahamsson's previous post about 
overruns.  I was quite wrong on that point.

Nick